It is widely known that propagation speed of an ultrasonic wave propagating through a sample gas is represented as a function of concentration and temperature of the sample gas. Supposing that an average molecular weight of the sample gas is M and the temperature is T [k], an ultrasonic propagation speed C [m/sec] in the sample gas is expressed by the following expression (1):
                    C        =                              kRT            M                                              [                  Expression          ⁢                                          ⁢                      (            1            )                          ]            
Here, k and R are constants (k: Ratio between constant-volume molar specific heat and constant-pressure molar specific heat, R: Gas constant). That is, if the ultrasonic propagation speed C [m/sec] and the temperature T [k] in the sample gas can be measured, the average molecular weight M of the sample gas can be determined. If such sample gas is composed by two components of oxygen and nitrogen, for example, it is known that k=1.4. Supposing that the molecular weight of oxygen is 32 and the molecular weight of nitrogen is 28, the average molecular weight M of the sample gas can be described as M=32P+28 (1−P) in a case of oxygen 100×P [%] (0≦P≦1) and nitrogen 100×(1−P) [%], and the oxygen concentration P can be determined from the measured average molecular weight M.
Further, supposing that the ultrasonic propagation speed in the sample gas is C [m/sec] and the flow velocity of the sample gas is V [m/sec], since an ultrasonic propagation speed V1 [m/sec] measured when the ultrasonic wave is transmitted in a forward direction to the flow of the sample gas is represented by V1=C+V and an ultrasonic propagation speed V2 [m/sec] measured when the ultrasonic wave is transmitted in a backward direction to the flow of the sample gas is represented by V2=C−V, the flow velocity V [m/sec] of the sample gas can be acquired by the following expression (2):
                    V        =                                            V              1                        -                          V              2                                2                                    [                  Expression          ⁢                                          ⁢                      (            2            )                          ]            
By multiplying the flow velocity V [m/sec] of the sample gas obtained as above by an inner area [m2] of piping through which the sample gas flows, a flow rate [m3/sec] of the sample gas can be obtained. Moreover, by volume conversion or time conversion, the flow rate can be easily obtained in [L/min].
Various apparatuses and methods for measuring the concentration and flow rate of the sample gas from the propagation speed or propagation time of an ultrasonic wave propagating through the sample gas using the above principle have been proposed. For example, Japanese Patent Laid-Open Publication No. H6-213877 describes an apparatus for measuring the concentration and flow rate of the sample gas by arranging two ultrasonic oscillators in the piping through which the sample gas flows opposite to each other and by measuring the propagation time of the ultrasonic wave propagating between the ultrasonic oscillators. In addition, Japanese Patent Laid-Open Publication No. H7-209265 and Japanese Patent Laid-Open Publication No. H8-233718 describe an apparatus for measuring the concentration of the sample gas by measuring the propagation speed or propagation time of the ultrasonic wave propagating through a sensing area in a sonic wave reflection method using a single ultrasonic oscillator.